1. Field of the Invention
The invention concerns an improved power converter device for direct current power supply to an electric arc furnace.
2. Description of the Prior Art
Electrical arc furnaces are used in melting and refining scrap metal.
The invention applies more particularly to supplying direct current to such loads, the electrical parameters of which are likely to vary widely and frequently.
FIG. 1 shows the oldest technique for supplying power to arc furnaces.
It entails feeding three-phase alternating current directly to the electrodes: a first step down transformer 1 supplies a set of busbars 2 distributing the current to one or more individual variable transformers 3 which reduce the voltage to the required level and feed the furnace 4 directly. The latter comprises a furnace chamber 5 receiving scrap metal 6 in its lower part and having three mobile electrodes 7 each connected to one phase of the secondary circuit of the step down transformer 3, so that an arc 8 is struck between the electrodes 7 and the scrap metal 6 to melt and subsequently refine the latter.
FIGS. 2 and 3 show the operating characteristics of an alternating current furnace of this kind. In particular, FIGS. 2 and 3 show the voltage (U.sub.arc)/current (I.sub.arc) characteristic, respectively and the reactive power (Q)/active power (P) characteristic for the various operating points (CC representing the short-circuit operating point, and F thru H representing the regime corresponding to melting of the scrap metal and refining of the scrap metal.
It can be seen that an alternating current supply furnace of this kind delivers substantially constant power (.DELTA.P is very low) but is subject to high variations in the reactive power (.DELTA.Q) and the current (.DELTA.I), operation being substantially at constant active power excluding short-circuit situations. In particular, the large variations in reactive power can cause high voltage fluctuations on the power supply system, injecting electrical interference into the latter, in particular in the form of "flicker", i.e. voltage fluctuations in the 0-30 Hz band causing lights to flicker.
For a few years there has been a trend to substitute direct current power supply for alternating current power supply by inserting an AC/DC converter between the downstream transformer and the electrodes.
Direct current power supply has two essential advantages from which all other advantages derive, namely:
The positive pole of the arc is the scrap metal itself, whether solid or melted, the mobile electrode (of which there may be one or more) constituting the negative pole. The dissipation of energy along the arc being non-linear and greater on the positive pole side, the scrap metal is heated more than the electrode. PA1 Current surges due to arc short-circuits, which occur very frequently during start-up and melting of the scrap metal, are limited by the converter.
This leads to reduced electrode consumption, reduced electrical interference fed back into the supply, especially flicker, and improved furnace productivity.
The direct current converters used until now are of the general type shown diagrammatically in FIG. 4.
The primary 9 of the downstream transformer feeds one or more secondaries 10, 11 each connected to a respective conventional "Graetz bridge" rectifier 12 using thyristors. One output terminal of each bridge is connected to a common mobile electrode 7 and the other terminal is connected, usually via a smoothing inductor 13, to a respective bottom or hearth electrode 14 in direct contact with the scrap metal 6.
The Graetz bridge 12 shown diagrammatically in the other figures is shown in detail in FIG. 5.
It includes two series 15, 16 each of three thyristors connected in "double parallel" (or "full-wave three-phase") mode with commoned cathodes and commoned anodes, respectively, and with the other electrode connected to the same (star or delta) multiphase voltage system; each series 15, 16 of thyristors is triggered with a respective common firing angle 51 or 52.
To limit the harmonics injected into the power supply system a plurality of six-pulse Graetz bridges is usually employed, each fed by phase-shifted secondaries. FIG. 4 shows two secondaries 10, 11 phase-shifted 30.degree. by a star-delta coupling feeding two Graetz bridges, but the solution can be generalized to three Graetz bridges (with phase-shifts of -20.degree., 0.degree., +20.degree.), four Graetz bridges (with phase-shifts of 0.degree., +15.degree., +30.degree. and +45.degree.), and so on, each Graetz bridge constituting an elementary six-pulse converter.
Although the remainder of this description considers only a system with two rectifiers, for example two Graetz bridges, it is to be understood that the invention can be generalized at will to a larger number of rectifiers fed by phase-shifted secondaries.
FIGS. 6 and 7 are homologous to FIGS. 2 and 3 in the case of a direct current device.
It can be seen that the device operates at constant current but with a widely variable active power, unlike the alternating current device. There are smaller, but still significant, variations in the reactive power Q. On average, the reactive energy consumption remains high, which usually implies the presence of a load regulator 17 for the downstream transformer and a relatively large compensator battery 18. In this respect the direct current device does not achieve any significant improvement compared to an alternating current device.
The reactive power variations (.DELTA.Q) from one operating regime to the other are nevertheless smaller than in the case of an alternating current power supply, whence smaller voltage fluctuations on the input side (on the power supply system) and reduced flicker. However, in the case of power supplies which are insufficiently rated, these voltage variations proportional to the reactive power consumption and inversely proportional to the supply short-circuit power often remain excessive, especially because of the flicker, and require the use of extremely expensive further correction means ("antiflicker" devices or Thyristor Control Reactors--TCRs), as for most alternating current solutions.
Considering the instantaneous reactive power consumption, however, note the very significantly lower direct current values, as current regulation by the converter is usually fast enough to limit the short-circuit current surge to a virtually negligible value.
One object of the invention is to remedy the respective drawbacks of alternating current solutions and direct current solutions by proposing a new direct current converter design which can significantly reduce reactive power consumption and significantly improve the voltage/current characteristics supplied to the direct current arc, whilst reducing the overall cost of the converter by simplifying the downstream transformer (the load regulator is no longer necessary) and significantly reducing the size of the compensator battery, typically by half.
It is explained below that the direct current converter of the invention can operate at substantially constant active power, as in the case of a furnace supplied direct with alternating current, but with low reactive power consumption and without any uprating of the converter or the associated transformer.
It is also shown that, by virtue of the teaching of the invention, it is possible to influence the reactive power/active power (Q/P) characteristic in a very simple manner in order to optimize it according to the context of use of the furnace.
It is possible, for example, to obtain a characteristic which minimizes the reactive power consumption for a given operating point or, alternatively, a characteristic which minimizes variations in reactive power about its mean value, especially in the case of low power rating supplies for which reducing the flicker is one of the main imperatives, both from the point of view of efficiency (minimizing interference injected into the power supply system) and in respect of the cost of the installation (by eliminating the "antiflicker" devices).
Also, the operating characteristics of the furnace can be chosen such that the reactive power consumption remains substantially constant despite any current variations, especially in the event of variations in the impedance of the load.